Saturday, 11 July 2020

NAVTEX Active Antenna

Ive finally come back to the antenna for the NASA Target NAVTEX receiver.

Ive mapped out the circuit diagram, and can now start working out how to fix it.

My suspicion of the lack of 518kHz was with the transistors, but ive had them off the board today and tested, and they check out ok.

Now, I know the PIC oscillator is not working. The original crystal was corroded, so has been replaced by one close but not exactly right. I would have expected it to work but give perhaps slightly off frequencies. I suspect that at some time the antenna feed has been reverse polarised, and this has killed the chip. It might well have killed the MC1496 as well.

So, the plan now is to remove the MC1496 and the PIC 12C508 from the board, plus lift any associated components that might affect the signal path, and see if I can get it working just on 518kHz, by bypassing the conversion circuitry. Im also going to remove the transformers and test them off-board to ensure their windings are intact, as some corrosion/dirt can be seen under one of them. Testing them on board is pointless as the DC circuitry is routed through them/across their pins.

If I can get that working, then I can work on the conversion side another time. 

Monday, 13 January 2020

Nasa Marine Target Navtex Pro Plus

Purely because I simply wanted one, I've eventually picked up a purpose built NAVTEX receiver. This is a Target NAVTEX Pro Plus unit from Nasa Marine. Second hand, and faulty, of course!


This is an older model, with external channel switching, as the people at Nasa Marine have been kind enough to supply me with a hand drawn copy of the receiver circuit diagram proving this. So the receiver itself can only handle the 518kHz broadcasts. To cope with the 490kHz National services, the active E-field probe antenna has frequency changing capability.

The receiver works - I have it connected to a short indoor length of wire, which allows it to receive some of the strongest signals. Its internal back-up battery is not in a very good way, but that's not much of a worry.

The antenna and Local/International switch box have issues. There is some corrosion on the antenna PCB - one tantalum bead capacitor has lost a leg - but it doesn't look too bad. There is a PIC micro-controller that looks to be serving as local oscillator, which has a lot of green gunk around and under it. One of the PCB traces looks rather poor as well. So long as the corrosion has not got into the transformers, repair shouldn't be too tricky. There is a destroyed track in the switch box.

So, a bit of linking out and cleaning up required!

Tuesday, 31 December 2019

XR2206 Function Generator kit, and Chinese project boxes

A number of items from the Far East have been trickling in over the holiday period, including 2N3906 PNP transistors, veroboard, audio transformers and project boxes. One such packet arrived today with five project boxes, snap-on lid types, for about £3. The quality is good, not perfect, but certainly adequate, and quite impressive for the cost! Two of these will become small Radiation detectors using the SI-19BG and the SBM-10 G-M tubes.

Another packet with two larger white boxes has also arrived. These are for the BOI-33 tube based detectors. The photo above shows the first version board fitted into the box. I need now to sort out a battery supply (waiting on coin cell holders - this will run on a 3V lithium cell), plus power switch, LED and neon indicator holes, and a sound hole for the loudspeaker, plus a row of holes along the side to allow better beta penetration to the tube.

Also for about £3 shipped, I received this little function generator kit. Actually a little smaller than I expected, the board was of reasonable quality, although the edges are a little rough, and some of the holes are a little too large.

Putting it together was very easy, although the instructions that came with it were pretty rubbish! The parts list at least was accurate.

Hard to see in the photo, but the clear acrylic case is etched with the names of the controls. The board is not bolted into the case, as the five(!) M3 nuts and bolts were too short to do so! However, the case itself is a reasonably tight fit and so there is little movement.  I've yet to test it though.

Tuesday, 24 December 2019

SBM-10 G-M tube

Yesterday I received an SBM-10 G-M tube from Russia. This tiny tube is just 6mm diameter and about 20mm long. I've tested it and it works well.

But, like the tiny SI-19BG, it has no easy way to connect to it. I'm wondering whether the socket for the SI-19BG can be made to work?

I've also realised that the voltage doubler I was working on yesterday wasn't complete! There should have been another diode, or series string. I need to test that out later.

Monday, 23 December 2019

Thinking the other way up!

All the Geiger circuits im working on, are intended for portable use, and so need the lowest voltage and lowest current supplies, to make them compact...

...but not the 1600V supply for the MST-17! This tube is far too delicate for portable use, and will only ever be used on the bench.. so why the heck am I trying to make it work from such low voltages? I know I can get 1500V from a 9V battery, even with the simple blocking oscillator circuit, so almost certainly I can get well above this, and hence enough to feed the regulator chain, with 12V!

As my bench PSU goes up to 30V, I can choose any voltage up to this, I'm not limited to a small battery for this tube!

Incidentally, the CCFL transformer I've been playing with has a primary of about 30 turns (found by unwinding one), so I can also rewind for a better turns ratio if I wish.

Voltage Doubler Oddity

Just prior to tidying up and the workshop 'closing' for christmas, I've been playing with the CCFL transformers taken from a defunct LCD monitor, a simple blocking oscillator, and a Cockcroft voltage doubler.
As this transformer is rated for 3kV AC, I have been very conservative and used several 1N4007 1kV diodes, plus the 3kV 4n7 capacitors from the same board as the transformer.

I already knew from previous tests that when fed with 9V, the blocking oscillator, which is extremely simple, draws a lot of current, around 150mA, and the 2N3904 transistor gets hot, so had been working with 3V where it only took around 17mA.

To this simple blocking oscillator/flyback topography, I added the voltage doubler, and was disappointed to get only 550V from it. Thinking that I had no need for the spread voltage handling of three diodes per half-section of doubler, I changed the circuit to the more common form with just two diodes - and got under 400V!

Now, this has me confused! Why should I get a higher output voltage when I have more diodes, and hence more forward voltage drop, in series?

So, with this thought troubling me, I decided to try it on 9V. As expected, the transistor did get hot, though not as hot as previously. At a current draw of 37mA (interestingly having the ammeter in circuit lowered the output voltage as well) I was now reading a little above 1600V - 1800V without the ammeter.

Clearly this circuit is not at all efficient - in fact loading the mid-point of the doubler with a neon bulb and 4M7 resistor drops the output first to 40V, then slowly lower until there is no output, and the current drawn jumps to over 60mA! This can probably be improved by adding a feedback path to regulate the output. I have seen a Geiger circuit using this type of transformer, operating from 1.2V, admittedly supplying a standard 400V tube, and using a zener feedback system.

Update - The thought occurs to me that, with just the two diodes forming the Cockcroft circuit, the reason for seeing just 550V may have been due to the combined reverse breakdown of the two diodes! These are rated 1kV, but the pair operating across the AC supply from the transformer could well have been operating in a controlled breakdown on each half-cycle, resulting in the observed 550V. With three in series for each side of the doubler, the combined breakdown voltage of each chain of three would be 3kV - much more than the applied voltages, and hence the more sensible 1500V output.

Saturday, 21 December 2019

The Owon SDS1102 100MHz Digital Storage Oscilloscope - Pt.2 - In Use

Before even starting acceptance testing, the first thing I did with this 'scope was to change the fuse in the kettle-lead plug for a 3A. Even this is big for the actual rating of the 'scope, but a damn sight safer than leaving a 13A fuse in there!

So with a safer fuse rating fitted, I powered up the 'scope, and set up my Geiger circuit as a test source.

Having folded out the two feet from underneath the scope, which weights about 1kg, and attached the scope probes, I first carried out the compensation adjustment to the probes, before connecting them to test points on the Geiger circuit.

In use, the 'scope is reasonably intuitive. Most controls are where you'd expect them and work as you'd expect them to work. The buttons have a reasonably positive action to them.

Blocking oscillator and flyback

Please forgive the poor colour rendering on the photos! I've used the same set of test points and measurements that I did with the DSO138 kit. So the first photo shows the trace of the blocking oscillator and the flyback pulse, the second is the flyback pulse expanded to show the inductor ringing.

Inductor ringing

The photo below shows the blocking oscillator with the FFT mode enabled, the top trace being the time domain, and the lower the FFT frequency domain.

FFT frequency domain function

Another option is to open a measurement window, which gives statistics of the signal being monitored. The menus, as with most menu driven devices, take a little getting used to. Most are accessed from a dedicated button on the main panel, then the options selected by the buttons beside the display.

Channel measurements option

And finally, both channels enabled, showing the G-M tube cathode connection, and the blocking oscillator. Here the side menu is shown as well for the Trigger options - the 'scope being set to trigger on rising edge 'normal' mode. The trigger signal in this case being the radiation event pulse shown in the middle of the screen.

Blocking oscillator lower trace, cathode pulses upper trace
All in all, so far, I'm quite happy with it. One thing missing though which would have made it very portable, is a DC supply connector! Despite its compact, lightweight portability - it still needs mains power.

Si-I-19BG G-M tube replaced and working!

Readers may recall that I bought an SI-19BG miniature Geiger-Müller tube from ebay seller 2015_Gurie in the Ukraine, and the delivered tube would not work. Some discussion with the seller resulted in an agreement to supply a replacement, on the understanding that if I couldn't make that one work (i.e. it was likely my fault not the tubes) I would return both.
Well, today at 15:50 - conveniently as I was acceptance testing the Owon oscilloscope, using the Geiger detector circuit as a signal source - the replacement tube was delivered.

After visually inspecting it and being happy it was undamaged physically, I connected it in place of the BOI-33 tube - and nothing happened! At this point I was a little miffed, but, I had a shiny new oscilloscope in front of me monitoring the cathode port - and yes, there were very faint, low level pulses! In fact, looking at the indicator LED of the Geiger circuit in the dark, I could see it occasionally flash dimly.

Ah-ha! So, this tube is doing something! So, what is amiss that prevents a good count pulse? The first thing to eliminate is the additional capacitance and resistance of the croc-clip leads used to connect the tube. With the first, faulty tube, I had an anode 4M7 resistor connected right to the tubes short anode lead, but this time, I was using the boards anode resistor, and a 30cm long croc-clip lead. So, I moved the anode lead of the tube to the anode clip on the board, and just used one croc-clip to secure it - and yes! Good, strong pulses and indications!

Of course, being such a tiny tube, its sensitivity is nothing like that of the BOI-33. Its background count is around 2cpm, compared with the BOI-33's 20cpm, but, it is a fraction of the size! It is also an end-window device, so is most sensitive from that end and little from around the body. This is fine though for a sub-miniature detector intended for sniffing out sources at charity shops and flea-markets! I think this tube will go into a circuit using 555 timers as HV generator and monostable indicator, but that will need to wait until I receive the project boxes from China.

NanoVNA and a curious kit

I spent much of yesterday Christmas shopping, which I hate (can't stand being in large crowds), but in the evening started to get to grips with the NanoVNA. But before that, I put together the perspex case kit that had arrived for the DSO138 oscilloscope.

Some instructions would have been nice!
This kit contained three more panels than I had expected, and absolutely NO instructions! So it took quite a bit of experimenting to discover the correct way to put it all together. From the photo above, the top right piece is the base - add to that the four long bolts and two nuts (to lock the bolts in position), the bottom left panel then goes over the main PCB but BEFORE the display! So, the display had to be removed for that stage. There were also four tiny nuts and bolt to secure the display to this panel. Once that is done, the two pieces with the large cut-out go on, these stack up and form the channels into which the button and switch levers fit. I found that these could move in use and make the switches stiff, so Ive added more nuts to secure them in position 8mm above the main board. Then the side panels go on, ensuring that the cut-outs align. This is easy for the BNC and power connector as the holes are different sizes, but the USB port cut-out is slightly off-set and needs some playing to find the right way around for the panel. The buttons and switch levers are now added, making sure the switch levers fit correctly over the switches. Finally, the top panel (top left in photo) goes on. A bit of jiggling is required here to align the eight studs in the side panels, and also the five button tops. The whole is then secured  with four dome nuts.

Actually looks quite good
 The completed case and kit actually looks quite good! I did find that the test point was now rather hard to access using the croc-clip probe, so used needle-nosed pliers to adjust this to make it easier. Other builders might want to take that into account during soldering and make a bigger loop!

An agreement was also found with the seller, concerning the faulty display, resulting an an acceptable partial refund.

So onto the NanoVNA. For such a tiny and low cost device, this thing is incredibly capable! However, when you first turn it on - it is rather daunting! It boots up in a full display mode - all four measurement channels, 50kHz to 900MHz range, making for a very busy and confusing display. Once you master the basics of driving the menu, which can be done via the touch-screen or the rocker control switch (which I actually found less reliable) and have turned off the traces you don't need, it becomes more manageable and easier to understand.

VSWR and Smith chart plot of a 2m rubber ducky antenna
So far the most I have done with it is to look at a 2m band rubber ducky antenna, but this is enough for finding your way around the menus, learning the modes and features etc. One thing I didnt expect, its charger port is a USB-C rather than the more usual micro-USB. Luckily it comes with a cable - as this is the only USB-C device I have!

I paid a little under £30 for this, shipped from the Far East of course. Compare that with the cost of an MFJ-259B, which doesnt display graphs, has to be manually adjusted for each frequency, is massively bigger, only goes to about 200MHz if that - and costs around 5x as much even second hand!

Thursday, 19 December 2019

The Owon SDS1102 100MHz Digital Storage Oscilloscope - Pt.1 - Unboxing

Despite a bit of a worry over the seller and the risk that it was coming from China and not within the UK, the 'scope arrive safe this morning. Maybe I had been a little hasty over the weekend!

So, first impression is of the packing. The outer packing was a thick polythene shipping bag. No additional padding so only half-marks there! I suppose it was a bit much to expect double boxing!

Inside the bag the scope was enclosed in a sturdy cardboard box with plastic carry handle.

As supplied in box

Opening the box revealed the 'scope wrapped in a thick packing bag and positioned centrally by two moulded packing pieces. Down one side was the poly zippybag containing the instruction booklet and a CD/DVD, down the other side the bag containing the probes. In a space at one end of the box was the mains cable and a USB lead.

Split bag containing probes

I did notice that the edge seam of the probe bag had split, but this isnt of much concern. Inside it was two 1x/10x probes, with removable spring clips, a trimming tool for adjusting the compensation capacitors, spare springs, and different ID colour rings. The probes themselves, which are about 1 1/2m long, already had yellow and blue ID rings on them. 

13A fuse for a 15W device! Change that!

If you buy one of these, or in fact any mains powered item these days - check the rating of the fuse in the plug! This 'scope has a specified power requirement of just 15W - yet the plug has a 13A fuse! Swap that for a more sensible fuse as soon as you can! At the very least a 3A fuse - but even this is much bigger than needed!

All the controls were in good order, straight etc, no scratches or marks anywhere on the unit. The unit itself is only about 3inch deep, and has fold out feet, and a fold out carry handle. It weighs very little.

On switching on, the unit displays a splash screen while booting up. This takes about 10-15sec.

Initial start up splash-screen

As its late, and Im very tired from work, the furthest I am going to test it tonight is to quickly connect one of the scope probe leads to the 5V square wave internal test signal port. The two BNC connectors for the input channels have protective plastic caps.

Internal test signal trace display

As can bee seen, the probe needs its compensation adjusting! But that is only to be expected. Over the next few posts I will look at using this 'scope, and some of its features. Of interest is the 'MATH' button, which brings up a menu of things it can do with the traces - including Fast Fourier Transforms.